The relatively high currents associated with charging large battery systems, such as battery systems for electrically powered vehicles, have been found to cause a number of annoyances including interference with radio transmissions and vibrations in magnetically active structures including vehicle body panels. Similar issues have also arisen when discharging battery systems, such as when operating an electrically powered vehicle. In such cases, the annoyances are attributed to oscillating magnetic fields that are induced by the charging and discharging currents (i.e., the exchange currents).
Although vehicle battery systems, and their associated electric motors, are typically configured as direct current (DC) systems, electrical currents flowing into and out of vehicle batteries typically exhibit non-steady characteristics. This is because control over the rate of charging or discharging a battery system (e.g., the charging voltage and the motor operation) is typically accomplished by breaking the current into pulses and modulating either the frequency of the pulses (i.e., the number of current pulses per unit of time), the width(s) of the pulses (i.e., the duration(s) of the current pulses), or, in three-phase systems, the shape of the pulses so as to satisfy a desired operating criteria. For example, a typical operating criterion for charging a battery system involves maintaining a constant Root Mean Square (RMS) or average voltage.
There being at least two degrees of freedom, (e.g., frequency and pulse width modulation, and sometimes phase modulation), a large number of possibilities exist for achieving a particular RMS or average voltage characteristic.
As demand has increased for electrically powered vehicles having increased range (i.e., increases in the amount of electrical energy that can stored in their battery system), and as demand has increased for reductions in the time required to recharge a battery system (i.e., for increases in the rate at which electrical energy can be restored to a battery), charging current necessarily is increased to meet these demands.
In addition to RMS or average voltage, other specific charging criteria may include avoiding limitations on battery heating or out-gassing. Accordingly, as a charging voltage approaches a desired set-point, a control may gradually reduce exchange current so as to avoid over-heating the battery or producing excessive levels of gas from the battery while continuing to recharge the battery at a desirable rate (e.g., in the shortest practical amount of time). Thus, modulation of the current can be performed to serve a number of goals, and some of those goals include providing benefits such as improving charging efficiency and speed, while ensuring charging safety and battery health.
As means have been developed for restoring a charge in a vehicle battery system at ever-increasing rates, and as methods such as pulse width modulation (PWM) have been implemented, a number of new problems have also been encountered. As mentioned above, these include the production of electromagnetic fields that interfere with radio transmissions and that produce annoying vibrations in vehicle structures. Attempts have been made to mitigate the problem of radio interference by shifting the frequencies of the modulated charging pulses.
To facilitate such mitigation efforts, however, the tuning frequencies of the receiver (i.e., the device seeking to avoid the interference) must be monitored, so that the frequency of the modulated charging pulses may be shifted to a frequency range falling outside the pass band of the receiver. Unfortunately, the requirement that the current exchange controller have real-time knowledge of the receiver's tuning frequency tends to increase the complexity of the charging system controller. Moreover, such solutions may be ineffective in addressing vibrations induced in body panels and other structures in the vehicle.
Others have attempted to reduce harmonic interferences by shaping the charging wave forms. Unfortunately, though, the magnitude of the exchange currents in vehicular applications and the relatively close proximity of the charging system to the broadcast receiving antenna tend to decrease the effectiveness of such filtering systems. In addition, where charging systems must be able to accommodate relatively high exchange currents, such as in vehicle charging systems, hardware requirements for filtering components (e.g., large high-voltage capacitors and high current ferrite magnetics) can be substantial.
Accordingly, it would be advantageous to have a simpler and more effective system and method for charging electric or hybrid vehicle batteries with reduced broadcast band interference and with reduced production of other annoyances such as vibrating vehicle structures. It would also be desirable to have a system and method for charging an electrical system of an electrically powered vehicle according to a modulation scheme configured for providing an exchange current that exhibits a desired current exchange characteristic while meeting one or more criteria based on annoyance.